Pulsed lasers are widely used in applications ranging from surgical devices to lithography systems for forming electronic microcircuits. There are a number of types of devices conventionally used for pulsed laser modulation. These include, for example, devices that deflect a portion of the laser light or cause diffraction, such as various types of acousto-optical modulators (AOM) and electro-optical modulators (EOM). Other types of modulators operate using light polarization state, such as a liquid crystal (LC) modulator. Still other types of pulsed light modulators operate by mechanical action, obstructing some variable portion of the laser beam, actuated by devices such as voice coils, piezoelectric actuators, motors, and servo devices, for example.
Each type of modulator that is conventionally used for pulsed laser modulation has some limitations. For example, mechanical devices operate only within a range of speeds. Some types of devices, such as acousto-optical modulators, are effective only over a range of wavelengths.
One area of particular interest for pulse modulation is in UV lithography. The drive toward continually improved resolution for microcircuit fabrication has stirred interest in using shorter wavelengths, with particular interest in light in the deep UV region, typically less than about 250 nm. However, modulation of a pulsed laser beam in this range presents a number of problems that defy conventional solutions. One problem relates to the spectral range, which exceeds the range of modulator devices such as AOM and EOM devices. For example, typical EOM materials such as KD*P (Potassium Dideuterium Phosphate) or KDP (Potassium Dihydrogen Phosphate) exhibit relatively strong absorption at the UV wavelengths, which results in a lower damage threshold of the material over this spectral range. This eliminates these devices as potential modulators for UV lithography applications.
Another problem relates to high pulse rates. UV light at suitable power levels is efficiently provided by excimer lasers, which can operate at pulse rates of 5-6 KHz or higher. This well exceeds the response speeds of mechanical light modulators that would otherwise be operable in the deep UV range. Thus, the combination of very short wavelengths and relatively high pulse frequencies defies conventional light modulation solutions.
Conventional approaches to pulsed laser modulation, constrained with respect to speed and flexibility, in turn limit the capabilities of UV lithography technology. It is illustrative to consider a few salient examples of conventional approaches:                UK Patent Application GB 2155647A entitled “Controlled Exposure” by Suzuki describes conventional control of pulse energy by controlling the drive signal to the laser itself. This allows variation in pulse-to-pulse intensity, which is desirable for applications such as UV lithography. However, this type of direct modulation of the laser drive current would only be suitable for pulsed lasers only at speeds below a few hundred Hz. Moreover, even if faster response were possible, it is generally more desirable to operate the pulsed laser at a constant power output rather than to attempt continuous modulation of the laser power itself. For these reasons, this type of approach does not provide a workable mechanism for high-speed pulse modulation.        U.S. Pat. No. 4,970,546 entitled “Exposure Control Device” to Suzuki et al. describes the use of a spinning filter wheel that interposes a set of filters in the optical path. Each filter provides a different attenuation level to the light, so that a fixed pattern of attenuation is provided to the laser beam. Such a solution can be used to provide modulation for earlier pulsed laser sources at 500 Hz.        However, this solution would not be usable with laser sources more recently developed that operate at 5-6 KHz. Moreover, the attenuation provided with this type of solution yields a discrete fixed set of power levels in a repeated pattern. Other disclosures showing similar spinning filter wheel solutions include those of U.S. Pat. No. 6,476,905 entitled “Step and Scan Exposure System Equipped with a Plurality of Attenuator Blades for Exposure Control” to Li and U.S. Pat. No. 5,119,390 entitled “Energy Amount Controlling Device” to Ohmori.        
Thus, although higher pulsed laser frequencies have been achieved in the past few years, lithography systems utilizing UV pulsed lasers have been unable to harness the additional potential this offers for enhanced exposure accuracy and processing speed. There is, then, a need for an apparatus that provides a high-frequency pulsed radiation beam wherein each pulse can be separately modulated.